This invention relates to grafts.
We have previously proposed that the flow pattern in arteries including the swirling pattern induced by their non-planar geometry operates to inhibit the development of vascular diseases such as thrombosis, atherosclerosis and intimal hyperplasia.
It is known from WO 95/09585 to provide a vascular prosthesis comprising a length of generally hollow tubing having openings at both ends thereof and including a non-planar curved portion so as to induce swirl flow in blood flowing through the curved portion. As explained in that publication, the swirl flow induced by skewing of the blood flow within the non-planar curved portion improves flow characteristics and reduces the potential for deposit build-up and vascular disease including intimal hyperplasia.
In WO 98/53764, there is disclosed a stent for supporting part of a blood vessel. The stent includes a supporting portion around which or within which part of a blood vessel intended for grafting can be placed so that the stent internally or externally supports that part. The supporting portion of the stent is shaped so that flow between graft and host vessel is caused to follow a non-planar curve. This generates a swirl flow, again to provide a favourable blood flow velocity pattern which reduces the occurrence of vascular disease, particularly intimal hyperplasia.
In WO 00/32241, there is disclosed another type of stent, in this case including a supporting portion around which or within which part of an intact blood vessel other than a graft can be placed. This supporting portion can prevent failure of the vessel through blockage, kinking or collapse. Again, the supporting portion of the stent is of a shape and/or orientation whereby flow within the vessel is caused to follow a non-planar curve. Favourable blood flow velocity patterns can be achieved through generation therein of swirl flow within and beyond the stent. Failures in blood vessels through diseases such as thrombosis, atherosclerosis, intimal hyperplasia can be significantly reduced.
Further aspects of how swirl flow is beneficial are explained in the above publications. It is further explained in Caro et al. (1998) J. Physiol. 513P, 2P how non-planar geometry of tubing inhibits flow instability.
In certain embodiments of the above publications the artificial or modified natural blood flow tubing is helical or part-helical. In the case of part-helical tubing, the prosthesis or the supported vessel may undergo less than one complete turn of a helix, for example less than one half or less than one quarter of such a turn.
In this specification, the “swept width” of a helix means the outer width of the helix when viewed axially of the helix. In cases where this swept width is relatively wide compared to the width of the tubing itself, the prosthesis or stent may be more bulky than is necessary or acceptable to induce the required swirl flow.
It has been proposed in WO 00/38591 to use internal helical grooving or ridging to induce helical flow. Similar proposals have been made in WO 97/24081 and EP 1127557 A1. However, the use of ribs or grooves in an otherwise cylindrical tube may not reliably induce swirl flow across the entire cross-section of flow. There may be a tendency for the flow nearer to the centre of the tube to follow a linear path, particularly for flows at higher Reynolds numbers. Furthermore, the ratio of the wetted perimeter to the cross-sectional area of a tube is increased by the provision of ridges or grooves. There is a departure from a circular cross-sectional shape. This may lead to increased flow resistance and a consequent pressure loss, and damage to blood vessels and blood cells and the development of pathology.
It is also proposed in WO 00/38591 to use a non-circular cross-section tube which is twisted. Again, however, a departure from circularity increases the ratio of the wetted perimeter to the cross-sectional area and will have disadvantages.
A further proposal in WO 00/38591 is to provide a circular-section tube bent into a cork screw shape. It is usual for the helix of a cork screw to have a clear gap down the middle, so that this proposed configuration would have a wide swept width compared to the width of the tubing, certainly more than two tubing diameters. The amplitude of the helix would be greater than one half of the internal diameter of the tubing and there would be no “line of sight” along the inside of the tubing. This proposal would therefore be relatively bulky and unsuitable for certain applications. A similar proposal is shown in FIG. 5 of WO 02/98325, the tubing having a helix with a large amplitude and again no “line of sight” along the inside of the tubing.
Various designs of elastomeric arterial graft prostheses are proposed in GB 2092894. In the version of FIG. 8 of that document, the interior surface is undulatory or corrugated, with different undulations either having parallel circumferential paths or joined in a “spiral” path. The corrugations are proposed as an alternative to reinforcement for improving the anti-kinking characteristics of the graft. In the case of the “spiral” corrugations which appear to be shown in FIG. 8, the angle of the corrugations to the longitudinal axis is relatively high, of the order of more than 70°. This is to be expected where the purpose of the corrugations is to improve anti-kinking or other structural characteristics, rather than for reasons relating to the nature of the blood flow through the graft. In fact, it is likely that the corrugations would tend to cause the flow to undergo sharp changes of direction leading to flow separation and the creation of stagnant near-wall regions.